In cultured hippocampal neurons, we found a ~35% decrease in spine number in APP−/− mice which was confirmed in CA1 neurons in 12-15 months old APP−/− mice but to a much lesser degree. Several reasons may account for this difference. A first, rather simple, explanation may be that heterozygous APP mice only carry one copy of APP, and as such there is only ~15% decrease in APP−/− mice compared to APP+/− mice. However, because we did not find such dose-response in cultured neurons, we focused on heterozygous instead of wild type animals in the in vivo studies to simplify both the breeding and analyses. Nevertheless, it is reassuring that both the in vitro and in vivo studies demonstrated similar reductions in dendritic spine density, thus providing further confidence that absence of APP has deleterious effects on dendritic spines in an age-related manner. The second possibility is the fact that the brain environment is imperfectly modeled in a culture system that lacks the normal complement of different cell types, such glial cells. Furthermore, the neuronal connectivity is also decreased in in vitro cultures compared to the brain in vivo.
